fantasia

fantasia (făntāˈzhə) [key] [Ital., = fancy], musical composition not restricted to a formal design, but constructed freely in the manner of an improvisation. In the 16th and 17th cent., however, the term designated a contrapuntal piece employing imitation and thus was one of the forerunners of the fugue. The term is also applied to improvisatory pieces based on earlier works, e.g., Vaughan Williams's Fantasia on "Greensleeves."